1. Field of the Invention
The present invention relates to the field of metal casting and in particular to a method and apparatus for the debinderization and removal of sand from metal castings.
2. Background and Prior Art
The manufacture of metal castings is typically carried out by pouring the molten metal into a sand mold wherein the interior cavity of the mold is suitably shaped to provide the exterior features and shape desired in the casting. Where the molding of interior features in the casting is required, a sand core is employed wherein the interior features of the casting are formed on the external surface of the core. The core and mold are generally made from a mixture of sand and a combustible organic binder which serves to hold the sand grains together and retain the desired shape of the core and/or mold. Following the casting and cooling of the metal part, the sand and binder must be removed from the casting. Sand removal can be accomplished mechanically or thermally.
In the mechanical method, the sand may be removed by physical means, such as shaking or chiseling which may lead to damage or scarring of the casting. If the sand is intended to be recovered for subsequent reuse, it must then be subjected to a heat treatment process to bum off the binder material.
The thermal method involves heating the sand (and casting) to a sufficient temperature to bum off the binder, freeing the sand and allowing it to fall off from the casting. Generally, in this method, heated gases are forced from a heat source to the metal casting, by mechanical circulation, driven, for example, by fans. As commonly practiced, the thermal method, using circulating fans, presents difficulties in that a considerable amount of the falling sand may be picked up in the gas stream and re-circulated thereby resulting in damage to the fans and other components. Furthermore, the rate of sand removal is a function of the rate of heating which, in turn, is a function of the heating capacity of the system and the amount of re-circulating gases. The amount of recirculating gas cannot be easily or conveniently modulated during normal operating conditions, which limits the system flexibility in handling variable loads.
U.S. Pat. No. 4,411,709 to Nakanishi discloses a method for the manufacture of aluminum alloy castings wherein, after casting, a portion of the core sands is mechanically removed and the casting is then heated to a suitable temperature to simultaneously bum out the binder and remove the sand while heat treating the casting.
U.S. Pat. No. 5,294,094 to Crafton et al. disclosed a process wherein the casting with the sand core is passed through a series of furnace units wherein heated air, circulated by fans, is used to heat the casting and bum off the binder. A further airflow is used to dislodge additional sand from the heated casting. The remaining sand may be removed in a subsequent quenching operation.
U.S. Pat. No. 5,354,038 to Crafton discloses a method and apparatus for removing sand core material from a casting and collecting and reclaiming sand. In the reclaiming process the sand is subjected to a fluidization step to aid in the removal of the binder.
U.S. Pat. No. 5,423,370 to Bonnemasou et al discloses a method for sand removal from a metal casting which comprises heating the casting to a temperature sufficient to pyrolyze the binder, in a fluidized bed of solid particles, such as sand.
U.S. Pat. No. 5,439,045 to Crafton discloses a process wherein the sand and residues of the casting material are collected and reclaimed. The process allows for re-use of waste gases from the casting operation.
It is an object of the present invention to provide an improved method and apparatus for the removal of sand from metal castings, especially light weight metal castings, such as aluminum or aluminum alloy castings.
It is a further object to provide a method and apparatus for the debinderization and removal of sand from metal castings that does not require the use of mechanically operated fans for the circulation of heated air.
The apparatus of the present invention includes a chamber wherein both the metal casting and the heat source are placed. Preferably, the metal casting is placed in a central zone of the chamber for debinderization and sand removal. The heat source comprises at least one hot air generator capable of generating a stream of high velocity heated gas at a temperature and oxygen level sufficient to debinderize the bonded sand. Preferably, two hot air generators are employed, positioned on opposite sides of the chamber at the approximate level of the metal casting(s) and directed to the metal casting(s). Various types of hot air generators may be employed, provided they are capable of generating a high velocity stream of heated gas having a temperature and oxygen content sufficient to combust the binder of the core and/or mold sand associated with the metal casting. Suitable hot air generators include, for example, gas burners, plasma rings, or electric heaters wherein an external blower may be used to force a stream of air through an electrically heated duct. Gas burners may be used, utilizing various fuels, such as, propane, natural gas, liquid fuels, such as oil, or powdered coal in a water slurry, or even solid fuels, such as coal. Preferably, the gas stream produced will have a velocity in the range of about 3000 to 5000 feet per minute. Typically, a gas stream suitable for combustion of the binder will have an oxygen content in the range of about 10 to 20 percent and a temperature in the range of about 700xc2x0 to 3000xc2x0 F., depending on the material being treated. In a preferred embodiment of the present invention, the hot air generators are high velocity gas burners having high excess air capability, for example, utilizing an air supply to the burner in an amount of about 100 to 700 percent, preferably about 150 to 300 percent excess of the amount required for complete combustion of the gas supplied.
The apparatus may be used in either a batch or continuous-type operation. Metal castings having core and/or mold sand to be removed may be placed in the chamber individually, or several at a time. Conveniently, the metal casting(s) may be placed in a carrier frame, preferably of open construction to allow sand, loosened from the casting, to pass through and be removed by gravity. For a continuous-type operation, the metal castings may be mechanically transported into the chamber by means of a suitable conveyor, for example, on a monorail. The stream of high velocity hot air not only supplies necessary oxidizing gases and the heat required to raise the temperature of the sand to the level necessary for debinderization, but also provides the necessary circulation of gases for efficient heat transfer and sufficient momentum for dislodging loose sand from the metal casting. As the sand is debinderized, it may be allowed to fall freely from the metal casting, optionally to a collection container below for subsequent treatment and/or reuse.
The chamber in which the debinderization is carried out includes an exhaust means to remove the spent gases in a direction that offers minimal or no interference with the falling sand. The exhaust means is positioned to permit the circulation of the heated gases around the debinderization zone prior to exiting the chamber. The exhaust means preferably includes a screen or grid through which the exhaust gases may pass freely, but which will serve to inhibit the passage of sand and other solid particles therethrough.
The apparatus of the present invention provides an important advantage over the prior art in that the rate of sand removal can be adjusted as a function of part size, amount of sand, and sand characteristics. This can be accomplished easily and cost effectively by varying the amount of heated air generated by the system rather than changing the capacity of re-circulating fans as in the prior art processes.